1. Field of the Invention
This invention relates to an electrophotographic photoreceptor which exhibits high durability and produces high quality images over a long period of time. The present invention also relates to an image forming method, image forming apparatus, and image forming apparatus processing unit.
2. Description of the Related Art
An electrophotographic method employed in copiers, fax machines, and laser printers and the like generally utilizes methods comprising a process represented by charging an electrographic photoreceptor, irradiating images, developing, followed by transferring a toner image to an image bearing member (transfer paper), fixing the toner image, and cleaning the surface of electrophotographic photoreceptor. In a number of fundamental characteristics required to be an electrophotographic photoreceptor utilizing such electrophotographic methods, for instance, (A) ability to charge a certain level of electrical potential in a dark environment, (B) small electrical charge dissipation in a dark environment, and (C) quick dissipation of electrical charge by light irradiation, may be mentioned besides having long-term reliability of image quality, low environmental impact, and low cost. The photoreceptors used conventionally in electrophotographic schemes include such commonly known methods for instance, a photoconductive layer mainly comprising selenium or a selenium alloy disposed on an electroconductive substrate, inorganic photoconductive material such as zinc oxide or cadmium sulfide dispersed in a binder, and utilizing amorphous silicon based material. However, more recently, organic photoreceptors have acquired a wide use for their cost efficiency, freedom of designing photoreceptors, and for their low environmental impact.
Among organic electrophotographic photoreceptors, photoconductive resins represented by polyvinyl carbazol (PVK), charge mobile complex types represented by PVK-TNF (2,4,7-trinitrofluorolenone), pigment dispersion types represented by phthalocyanine binders, and multi functions that use an electrical charge generating substance and electrical charge transporting substance in combination are known. Specifically, a function-separating type photoreceptors are particularly outstanding in terms of sensitivity, durability, and stability.
The latent electrostatic image formation mechanism of these function-separating type photoreceptors is as follows. When light irradiated after charging the photoreceptor, the light passes through the transparent electrical charge transporting layer and is absorbed by the electrical charge generating substance in the electrical charge generating layer, the electrical charge generating substance which absorbed light generates charge carriers, then, the charge carriers are injected into the electrical charge transporting layer, and moved in the electrical charge transporting layer along an electric field generated by the charging, and a latent electrostatic image is formed by the neutralization of the electrical charge on the photoreceptor surface. In a function-separating type photoreceptor, the use of a combination of an electrical charge transporting substance having high mobility and mainly absorbing the ultraviolet portion and an electrical charge generating substance having high molecular efficiency and absorbing mainly the visible portion is known and useful.
However, most of the organic electrical charge transporting substances intended for use in organic electrophotographic photoreceptors utilizing an electrophotographic method are low molecular-weight compounds, which do not form films when used alone, and therefore, they are usually used as mixtures and dispersants of inactive polymer. However, electrical charge transporting layers comprising a low molecular-weight electrical charge transporting substance and an inactive polymer are generally soft, and accordingly, suffer shortcomings such as easy ware out by mechanical load exerted on the surface of the photoreceptor due to the developing system or cleaning system used repeatedly in an electrophotographic process. Actually, the thickness of the photoreceptor film decreases due to wear (hereinafter called film erosion), thereby sensitivity deteriorates and charging properties deteriorate, image density reduces, abnormal images are produced due to texture smudging and the like, and leads to shorten a life of photoreceptor, making replacement necessary.
In recent years, as image forming apparatuses have become increasingly smaller, progress have been made in making diameters of photoreceptors smaller, as well as trend towards higher machine speeds and for realizing maintenance-free operation created a strong demand for producing highly durable photoreceptors. In order to achieve higher durability, the first target is to improve wear resistance, as mentioned earlier. Examples in the art regarding this purpose include (1) use of hardening binder in a surface layer (Japanese Patent Application Laid-Open No. S56-48637/1981), (2) improvement made in binder resin of the photoconductive member (Japanese Patent Application Laid-Open No. H5-216250 and 2000-98644), (3) use of polymer type electrical charge transporting substance (Japanese Patent Application Laid-Open No. S64-1728/1989 (published), and (4) dispersing an inorganic filler in the surface layer (Japanese Patent Application Laid-Open No. H4-281461/1992 (published).
Of these foregoing arts, (1) the art that uses a hardening binder, because of poor miscibility with the electrical charge transporting substance, there are impurities such as polymerization starters and unreacted residual groups, due to which the electric potential in the exposed portion rises and image density decreases. (2) as method of improving the binder resin, usage of polyester resin containing an alkylene terephthalate structural unit may be mentioned. However, such method derives new issues such as occurrence of toner filming caused by lowered softening temperature, deterioration in solubility to coating solvents, in addition to unattained improvement in resistance to wear which attributes to composition ratio of the low molecular electrical charge transporting substance. And with (3), by utilizing a polymer type electrical charge transporting substance, some degree of improvement in wear resistance may be realized, durability is not sufficient, and there are manufacturing difficulties such as material polymerization and in refining, thus high purity is difficult to achieve, and moreover, high viscosity in coating liquid. With (4), an inorganic filler used as dispersion, as compared to a photoreceptor which uses an ordinary low molecular-weight electrical charge transporting substance dispersed in an inactive polymer, has high durability and the electrical characteristics in repeated use is maintained, thereby given some attention.
Nevertheless, photoreceptors dispersed on its surface layer with an inorganic filler suffers such shortcomings as image smearing caused by oxidizing gasses such as ozone and Nox derived from electrical charger and the like during the repeated use. This phenomenon is caused by several factors. The latent image on the photoreceptor surface formed by charging-exposure diffuses, and triggers electric potential contrast to decline, and causes image smearing. Such image smearing appears in the developed toner image as a thickening of written characters and thin lines, a deterioration in resolution, and lack of density in intermediate tones. In notable cases written characters and drawings are indistinguishable. Hence image smearing is a phenomenon caused by decrease in resistance of the photoreceptor surface. Image smearing in a photoreceptor dispersed with an inorganic filler is believed to be caused by a number of factors. For instance, due to dispersion of the inorganic filler, durability is improved, thereby the renewal of the photoreceptor surface due to erosion cannot keep up with the permeation of the oxidizing gases. Another instance is that toner and paper dust produces a so called filming on the irregularities built up on the surface due to inorganic filler dispersion, to which the oxidizing gases adhere. Third instance is that non-insulative fillers or surface treated filters are used to stabilize the electrical properties, thereby the resistance of the surface layer is weakened. As measure for preventing such image smearing during repeated use, there are methods such as preventing moisture adherence by warming the photoreceptor with a heater, or releasing the oxidizing gases. However, these methods cause problems, in that they are time consuming at the start up of the equipment and require large energy.
Another measure is the method of adding an antioxidant to the surface layer (Japanese Patent Application Laid-Open No. H8-292585/1996 (published)), but there is a little effect in adding only a small amount, and electrical characteristics deteriorate when a large amount is added. There is also a method in which a polyarylate resin is contained in a surface layer binder resin (Japanese Patent Application Laid-Open No. H8-248666/1996 (published)), however, performance do not reach the level of satisfaction using ordinary polyarylate resins consisting of a single component as of present. To date, no means have been found for satisfactorily resolving the problem of image smearing during repeated use.